The (Al,Ga,In)N material system includes materials having the general formula AlxGayIn1-x-yN where 0≦x≦1 and 0≦y≦1. In this application, a member of the (Al,Ga,In)N material system that has non-zero mole fractions of aluminium, gallium and indium will be referred to as AlGaInN, a member that has a zero aluminium mole fraction but that has non-zero mole fractions of gallium and indium will be referred to as InGaN, a member that has a zero indium mole fraction but that has non-zero mole fractions of gallium and aluminium will be referred to as AlGaN, and so on. There is currently considerable interest in fabricating semiconductor light-emitting devices in the (Al,Ga,In)N material system since devices fabricated in this system can emit light in the blue wavelength range of the spectrum. Semiconductor light-emitting devices fabricated in the (Al,Ga,In)N material system are described in, for example, U.S. Pat. No. 5,777,350. There is also interest in fabrication electronic devices, such as high-performance transistors, in the (Al,Ga,In)N material system.
FIG. 1 is a schematic view of a typical semiconductor laser device (or laser diode—“LD”) 10 fabricated in the (Al,Ga,In)N material system. The device is able to emit light in the blue wavelength range.
The laser device 10 of FIG. 1 is grown over a substrate 1. In the laser diode 10 of FIG. 1 the substrate 1 is a sapphire substrate.
A buffer layer 2, a first cladding layer 3 and a first optical guiding layer 4 are grown, in this order, over the substrate 1. In the embodiment of FIG. 1 the buffer layer 2 is a n-type GaN layer, the first cladding layer 3 is an n-type AlGaN layer, and the first optical guiding layer 4 is an n-type GaN layer.
An active region 5 is grown over the first optical guiding layer 4.
A second optical guiding layer 7, a second cladding layer 8 and a cap layer 9 are grown, in this order, over the active region 5. The second optical guiding layer 7 and second cladding layer 8 are of opposite conductivity type to the first optical guiding layer 4 and first cladding layer 3; in the laser device 10 of FIG. 1 the first optical guiding layer 4 and first cladding layer 3 are n-type so the second optical guiding layer 7 and second cladding layer 8 are p-type layers. In the laser device of FIG. 1 the second optical guiding layer 7 is a p-type GaN layer, the second cladding layer 8 is a p-type AlGaN layer, and the cap layer 9 is a p-type GaN layer.
The structure of the active region 5 of the laser device 10 is not shown in detail in FIG. 1. In general, however, the active region 5 will be either a single quantum well (SQW) active region having one quantum well layer disposed between first and second barrier layers, or a multiple quantum well (MQW) active region having two or more quantum well layers with each quantum well layer being disposed between two barrier layers. The quantum well layer(s) may be, for example, layers of InGaN, AlGaN or AlGaInN.
High quality heterostructure field effect transistors (HFET) and high electron mobility transistors (HEMT) typically require a combination of large electron sheet densities and high electron mobilities in the channel region. One approach to obtaining the necessary electron sheet concentration and electron mobility is to incorporate an electron gas region into the device. Electrons in an electron gas can have a much higher mobility than those in a bulk semiconductor crystal as a result of fewer interactions (or scattering) with host or dopant atoms.
It may also be desirable to incorporate an electron gas region into a nitride semiconductor optoelectronic device, as is described below.
As is known, an electron gas region may consist of, for example, a potential well in which electrons can accumulate to form an electron gas. Depending on the shape and thickness of the potential well, the electrons in the electron gas may be confined in the direction perpendicular to the plane of the quantum well and may be free to move only in the two dimensions parallel to the plane of the quantum well; in this case, the electron gas is known as “a two-dimensional electron gas” (2DEG). Alternatively, the electrons of the electron gas may be free to move in all three dimensions in which case the electron gas is known as “a three-dimensional electron gas” (3DEG). Electrons in an electron gas can have a much higher mobility than those in a bulk semiconductor crystal as a result of fewer interactions (or scattering) with host or dopant atoms.
Jeganathan et al. have reported, in J. Appl. Phys. 94 (2003) p 3260, that an AlGaN/GaN heterostructure can be used to achieve a 2DEG with a sheet carrier concentration of up to 5×1013 cm−2 without intentional doping of the heterostructure. This is well in excess of the sheet carrier concentration achievable in other III-V systems such as the AlGaAs/GaAs system, and this is mainly due to the five-times larger piezoelectric polarisation of a strained AlGaN layer compared to AlGaAs and the very large spontaneous polarisation (polarisation at zero strain) in wurtzite III-nitrides compared to other III-V materials. However, the method of Jeganathan et al. requires the use of an AlGaN layer that has a very high aluminium mole fraction, possibly even use of an AlN layer, in order to obtain a 2DEG having a sheet carrier concentration of 5×1013 cm−2. It is generally undesirable to incorporate a layer of AlN or AlGaN with a high aluminium mole fraction into a device, since this can lead to excessive strain within the device, to the formation of dislocations, and to unwanted impurity incorporation.
Co-pending U.S. patent application Ser. No. 10/974,348 (Co-pending UK patent application No. 0325100.6) discloses a semiconductor light-emitting device fabricated in a nitride material system and having an active region disposed over a substrate. The active region comprises a first aluminium-containing layer (for example an AlGaN layer) forming the lowermost layer of the active region, a second aluminium-containing layer forming the uppermost layer of the active region, and at least one InGaN quantum well layer disposed between the first aluminium-containing layer and the second aluminium-containing layer.
Jena et al. report, in Appl. Phys. Lett. 81(23) 2002 p 4395, the use of AlGaN layers with a graded aluminium mole fraction to produce a 3-D electron gas or slab. This method produces an electron gas having a maximum sheet electron concentration of 9×1012 cm−2.
Ibbetson et al. discuss, in Appl. Phys. Lett. 77(2) 2000 p 250, the source of electrons in an electron gas at an AlGaN/GaN interface. They identify surface states as major source of electrons and predict a maximum sheet electron concentration of 4.8×1013 cm−2 for an electron gas at an AlGaN/GaN interface. They make no mention of how to form the surface states.
Mkhovan et al. describe, in J. Appl. Phys. 95(4) 2004, p 1843, the formation of a quasi-2DEG with an electron sheet concentration of approximately 5×1013 cm−2 in the AlGaN/GaN materials system. They state that the sheet electron concentration is governed by polarisation induced charge and is influenced by interface diffusion. They give few details of how a 2DEG is formed or of the source of electrons for a 2DEG.
Jeganathan et al. (above) have reported, in J. Appl. Phys. 94 (2003) p 3260, that an AlGaN/GaN heterostructure can be used to achieve a 2DEG with a sheet carrier concentration of up to 5×1013 cm−2 without intentional doping of the heterostructure. However, there is no teaching of incorporating an AlGaN/GaN heterostructure into an optoelectronic device to provide an electron gas region and improve the optical efficiency of the device.
U.S. Pat. No. 6,515,313 discloses a light emitting device in which dipole reducing methods (such as interface grading or interface doping) are used to reduce any polarisation induced fields across the active region of the device so as to improve optical efficiency. In the present invention, however, dipoles and a polarisation field are specifically introduced to form an electron gas.
U.S. Pat. No. 6,541,797 discloses a light emitting device fabricated in a nitride materials system and having a 2DEG (with a sheet carrier concentration of less than 5×1013 cm−2) formed in the active region, on its p-type side. The 2DEG region acts as collecting region for efficient electron-hole recombination. In the present invention, in contrast, the electron gas region is situated outside the active region, and is on the n-type side of the active region.
U.S. Pat. No. 6,614,060 describes a light emitting device containing a wide InGaN layer situated below the active region, for electron capture and resonant tunnelling of electrons into the active region. There is no indication that an electron gas is formed anywhere in the device.
Kim et al. report, in Phys. Stat. Sol. (a) 201, (2004), p 2663, the use of an electron tunnelling barrier in a nitride LED for improved optical efficiency. The electron barrier is provided by an AlGaN layer with a thickness of 2 nm located in or just beneath the active region of the LED. The AlGaN layer is reported to reduce hot electron overflow out of the active region. The formation of an electron gas at the interface between the AlGaN tunnelling barrier and underlying GaN is not reported or suggested by Kim et al. Moreover, according to Smorchova et al in J. App. Phys. 86 (1999), p 4520, such a thin AlGaN layer would not result in the formation of a 2DEG.
Luo et al. report, in J. Elec. Matl. 30(5) 2001, p 459, an improvement in photoluminescence emission intensity from InAs quantum dots (QDs) coupled to a 2DEG. The 2DEG is thought to act as an electron reservoir for the QDs, through wave-functions of electrons in the 2DEG overlapping with the QDs. In the present invention, however, the electron gas is outside the active region and so the wavefunction of electrons in the electron gas is not coupled to the active region.